The present invention relates to a clamping mechanism for securing a semiconductor wafer during wafer handling. More particularly, the present invention is directed to a clamping mechanism that securely clamps a semiconductor wafer near the distal end of a robot arm.
A wafer is the base material, usually silicon, used in semiconductor chip or integrated circuit fabrication. Typically, the wafer is a thin slice of base material cut from a silicon ingot or xe2x80x9cboule.xe2x80x9d Each 8 inch (200 mm) production wafer is approximately {fraction (1/30)} inch (0.85 mm) thick and has a diameter of approximately 8 inch (200 mm). Because of the nature of the base material and the thinness of each slice, the wafer can easily be damaged through mishandling.
Wafers are typically processed into semiconductor chips by sequentially exposing each wafer to a number of individual processes, such as photo masking, etching and implantation. Modern semiconductor processing systems include cluster tools that aggregate multiple process chambers together, where one or more of the individual processes are performed in each chamber. These process chambers may include, for example, degas chambers, substrate pre-conditioning chambers, cool down chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, etch chambers, or the like.
Typically, these process chambers surround a central chamber housing a central wafer handling robot, which manipulates the individual wafers. The cluster tool also typically includes a cassette in which multiple wafers are stacked before and after semiconductor fabrication. The wafer handling robot has access to the multiple process chambers and the cassette through load ports coupling each chamber and cassette to the central chamber. During operation the wafer handling robot repetitively transports wafers from one chamber to another, or to and from the cassette. Processing times can range from a few seconds to a few minutes, depending on the specific type of process that is required. Furthermore, the cluster tool forms a sealed environment, generally at vacuum, that is controlled to limit potential contamination of the wafers and to ensure that optimal processing conditions are maintained. Examples of cluster tools can be found in U.S. Pat. Nos. 5,292,393; 5,764,012; 5,447,409; 5,469,035; and 5,955,858, all of which are incorporated herein by reference.
The high costs associated with manufacturing semiconductor devices together with the demand for lower consumer prices has brought about a push to increase fabrication efficiency. In order to increase fabrication efficiency, equipment makers now seek to reduce processing equipment footprint, cost of ownership, and power consumption, while increasing cluster tool reliability and throughput.
The throughput for a particular cluster tool is mainly dependent on the number of process chambers and the time required for a process chamber to service each wafer. Ideally, the maximum throughput for which a cluster tool is capable is:       Maximum    ⁢          xe2x80x83        ⁢    ideal    ⁢          xe2x80x83        ⁢    cluster    ⁢          xe2x80x83        ⁢    tool    ⁢          xe2x80x83        ⁢    throughput    =            N      ·              (                  60          t                )              ⁢          xe2x80x83        ⁢    wph  
where
N=number of process chambers;
t=time required to process one wafer in minutes; and
wph is the number of wafers per hour that a cluster tool is capable of processing.
In order to calculate the actual throughput, the material handling issues must also be considered. The actual cluster tool throughput will always be less than the ideal throughput because of time lost in wafer transfers through the central chamber. For example, once a process chamber completes a process sequence on a wafer, it may take as much as 30 seconds for the central wafer-handling robot to replace the processed wafer with another unprocessed wafer. Since the time required for the robot to swap wafers detracts from the time in which the process chamber is actually processing wafers, minimizing the wafer swap or handling time at each process chamber will have a direct positive impact on the total throughput of the cluster tool.
A high throughput can be achieved in a number of ways. First, duplicate chambers can be provided. This, however, substantially increases the cost and complexity of each cluster tool. Second, additional wafer handling robots can be provided in each cluster tool. Again, this increases the cost and complexity of each cluster tool. Third, the speed of any individual process can be increased. However, although each process is always being improved upon, each process is typically completed in as short a time as is currently possible. Finally, the handling speed of each wafer by the wafer handling robot can be increased, i.e., the wafer handling robot must rotate and extend as fast as possible without causing the clamped wafer to slip during transport. Slip occurs when the robot accelerates the wafer such that its inertia overcomes the coefficient of static friction between the wafer and the blade material, causing undesired wafer movement and resulting in wafer misalignment and possibly the generation of unwanted particles.
Increasing the handling speed, however, is subject to a number of constraints, such as: each wafer must be securely grasped or clamped by the wafer handling robot in the minimum amount of time; the clamping of the wafer must be firm, but not overly so, so as not to damage the fragile wafer; the clamping and placement of each wafer must be precise and accurate, any misplacement might negatively impact the process and/or damage the wafer; transfer between chambers, or into or out of the cassette, must be smooth so that the wafer does not undergo any unnecessary stress, or in the worst case dislodge from the clamping mechanism; the clamping mechanism must be heat resistant, as some of the processes may expose the clamping mechanism to high temperatures; the clamping mechanism must not introduce into the closed environment any particulates or contaminants that can ultimately damage the wafer or semiconductors (it has been found that particulates as small as the critical dimension or line width of a semiconductor device, currently 0.18 xcexcm, can damage the integrity of an integrated circuit formed on a wafer); the wafer clamping mechanism should be able to automatically center a misplaced wafer; and finally, the wafer clamping mechanism must not apply a static electric field to the wafer, which might discharge and damage the semiconductor devices being fabricated.
Of the abovementioned ways of increasing wafer throughput, increasing the handling speed of each wafer is the most practical and cost effective. Therefore, to address the above criteria, a more robust and better designed wafer clamping mechanism is required.
Currently, in order to minimize the time required to move a silicon wafer from one place to another, many atmospheric wafer-handling robots employ vacuum or electrostatic chucks to hold wafers firmly in place on the robot end-effector during transport. However, since vacuum chucks rely on a pressure differential to create the chucking force to hold the wafer in place, they typically cannot be used in vacuum robot applications. In addition, electrostatic chucks are difficult to incorporate in vacuum robots for a number of reasons including vacuum feed-through design complexities, limited performance, reliability, and cost. As a result, vacuum robots typically rely only on frictional forces between the wafer and robot end-effector to prevent relative motion during transport; and a robot must therefore move slowly enough that the wafer does not move relative to the end-effector. This can significantly impact wafer swap time.
Alternatively, some vacuum robot end-effectors or wafer carrying blades, such as those disclosed in U.S. Pat. No. 5,746,460, are designed with deep wafer carrying pockets or blades that are just slightly larger in diameter than the wafer itself. These tight pockets prevent the wafer from moving on the end-effectors or blades during transport. Also, although the wafer transport robot is required to place wafers with extreme precision, there is no guarantee that the wafer will be precisely placed on the robot end-effector when it is picked up initially. A deep blade or pocket with tapered sides is sometimes used to provide a mechanical centering effect.
A number of prior art devices have attempted to clamp the wafer using active clamping mechanisms. One such prior art device 100 is shown in FIG. 1A which is derived from U.S. Pat. No. 5,955,858. This shows a bottom view of a wrist assembly 102 with its bottom cover plate removed. Clamp fingers 108, shown extended from the wrist assembly 102, engage a perimeter of a wafer 104 to clamp the wafer 104 onto a wafer carrying blade 106. The wafer 104 is held between the fingers 108 and a blade bridge 110 under forces applied by a pair of parallelogram springs 112. Parallelogram springs 112 bias the fingers 108 toward the wafer 104.
The wrist assembly 102 is coupled to the distal end of frog-leg type robot arms 114 of a wafer handling robot. During extension of the robot arms 114, i.e., when the robot arms are drawn toward one another in the direction shown by the arrows in FIG. 1A, a rotation is imparted on pivots 116, which in turn rotate cogs 118. The cogs 118, in turn, engage with the fingers 108 to retract the fingers 108 away from the wafer 104. Therefore, the wafer 104 is released when the robot arms 114 are extended and clamped when the robot arms 114 are retracted. If the fingers were directly attached to the cogs 118, then the clamping force would depend on the motion characteristics of the robot, for example, the speed of extension and retraction of the robot arms 114. In this device the clamping force of the fingers can be set independently by controlling the stiffness of the parallelogram spring 112.
A drawback of wrist assembly 102 is that the parallelogram springs 112 are easily deformed by out-of-plane forces, causing the clamping force direction to deviate from the norm. This leads to unreliable clamping and potential particle contamination caused by friction between the fingers and the wafer. Furthermore, a low cycle life of the parallelogram springs 112 (approximately 1 year or 10 million spring cycles) has been found to be inadequate.
In addition, the wrist assembly 102 does not provide for clamping a wafer that is not centered correctly. If the spring is deformed, the capture pocket, i.e., the total area in which the clamping mechanism can capture a wafer, could easily change, thereby, reducing the tolerance of the wafer handling system to deviations in the position of the wafer during transfer to and from each process chamber.
It has also been found that manufactured parallelogram springs are highly sensitive to manufacturing defects and mishandling before, during, and after installation, leading to unreliable clamping. Furthermore, the manufacturing process for the springs requires an electropolish step, which cannot be controlled reliably. Finally, any kinks in a spring caused by mishandling, lead to stress concentration points that reduce the fatigue life of the spring.
Another prior art clamp wrist assembly is disclosed in U.S. Pat. No. 6,155,773. A partial bottom view of this prior art clamp wrist assembly 120 with its bottom cover plate partially removed is shown in FIG. 1B. This clamp wrist assembly 120 comprises a lever assembly 122, a flexure member 124, and a pair of clamp fingers 126 that engage a wafer 130. Leaf springs 128 bias the flexure member 124 against the wafer 130. When the clamp wrist assembly 120 is in its extended position, a translational member 132 engages a first lever 134 to retract the fingers from their clamping position. However, this wrist assembly 120 does not clamp a wafer that is not centered correctly. Moreover, space limitations prevent this clamp wrist assembly 120 from being implemented on an opposed dual blade robot.
Finally, another prior art wafer holder is disclosed in U.S. Pat. No. 5,810,935. A partial bottom view of this wafer holder 140 with its bottom cover plate removed is shown in FIG. 1C. Wafer holder 140 includes two rotatable holding means 142 for holding rounded edges of wafer 144, and an electrical actuating means 146 for operating the holding means 142. Tension springs 148 bias the holding means 142 towards the wafer 144. Introduction of the electrical actuating means 146 not only introduces additional complexity and cost into the system, but also leads to more potential areas of particle generation and potential electrical fields, both of which might damage the wafer.
In light of the above, there is a need for a wafer clamping mechanism that securely clamps a wafer for speedy handling, meets the abovementioned criteria, and addresses the drawbacks presented by the prior art.
The present invention is an apparatus for securely holding a silicon wafer on a vacuum robot end-effector or blade by mechanically clamping the wafer during transport. This approach not only allows the robot to move more quickly, but can also mechanically center the wafer during transport.
The wafer clamping apparatus of the present invention includes a cam rotatably coupled to a base plate. The cam is configured to couple with a robot arm. The clamping apparatus also includes a rotating clamp mechanism rotatably coupled to the base plate about a single fixed point. A biasing mechanism, coupled to the rotating clamp mechanism, urges the rotating clamp mechanism to a clamped position. The rotating clamp mechanism is configured to interact with the cam to engage and disengage the rotating clamp mechanism from the clamped position. The rotating clamp mechanism preferably comprises a hub rotatably coupled to the base plate and a clamping arm and cam follower extending from the hub. The clamping arm is configured to clamp a wafer when the rotating clamp mechanism is in the clamped position, while the cam follower is configured to interact with the cam.
The wafer clamping mechanism preferably also includes a wafer carrying blade coupled to the base plate and a robot arm coupled to the cam. In use, the rotating clamp mechanism engages when the robot arm is retracted and disengages when the robot arm is extended.
The clamping mechanism reliably increases throughput while reducing cost. The clamping mechanism also provides the benefit of passive wafer centering, versus more costly active center finding methods, thereby eliminating the potential for failure due to variances in wafer placement. Furthermore, the clamping mechanism can be also be customized such that the wafer contact is made slowly and smoothly, even if the angle between the arm and base plate changes quickly and abruptly.